Answer
$(x-1-z)(x-1+z)$
Work Step by Step
Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the difference of two squares, then,
\begin{array}{l}
(x-1)^2-z^2
\\=
[(x-1)-z][(x-1)+z]
\\=
(x-1-z)(x-1+z)
.\end{array}
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